The microwave plasma CVD process employs a microwave to excite and decompose a material gas to generate a plasma which forms a thin film on a substrate or a base. The microwave plasma CVD process is finding use in a wide range of applications from the fabrication of semiconductors to the synthesis of thin films of diamond. The microwave plasma CVD process is highly practical as it can achieve desired film qualities, film thicknesses, and film yields with high reproducibility.
One conventional microwave plasma CVD apparatus is illustrated in FIG. 6 of the accompanying drawings. In FIG. 6 , the microwave plasma CVD apparatus includes a microwave waveguide 2a having an end 2b and an E plane which is perpendicular to an electric vector, and a cylindrical reaction tube 6 of quartz extending through the waveguide 21 a perpendicularly to the E plane at a position adjacent to the end 2b of the waveguide 2a. A shorting plunger 5 is mounted in the end 2b of the waveguide 2a.
The shorting plunger 5 serves as a short-circuiting device for fully reflecting a microwave which has traveled in the waveguide 2a. When the shorting plunger 5 is actuated, a plasma generated in the reaction tube 6 is moved several cm only in the direction of travel of the microwave, thereby finely adjusting the position where the plasma is generated.
The microwave plasma CVD apparatus is usually adjusted in advance to position of the center of the plasma at the central axis of the reaction tube 6. When the shorting plunger 5 is adjusted, the position where the plasma is generated can be moved one-dimensionally in the direction of travel of the microwave. However, it is not possible to move the plasma in a direction normal to the direction of travel of the microwave, and hence to adjust the position of the plasma two-dimensionally.
Since the reaction tube 6 of quartz is transparent, it does not inherently absorb or reflect the microwave. However, if any by-product is deposited on the inner wall surface of the reaction tube 6 during a reaction process, then the by-product deposit absorbs or reflects the microwave.
Any changes in the cross-sectional area of the waveguide 2a or the presence of electric charges or currents in the waveguide wall may interrupt electric and magnetic fields, causing the microwave to spread in perpendicular and oblique directions as well as to travel straight.
If the travel of the microwave in the reaction tube 6 is disturbed, then the distribution of electric field intensities is made uneven, and so is the plasma. When a thin film of diamond, for example, is synthesized by CVD under such a condition, the plasma and the base on which the thin film is to be formed have an ununiform distribution of temperatures. As a result, the quality of the diamond film, the rate at which the diamond film is synthesized, and the thickness of the diamond film become irregular. In the case where the synthesized diamond film is used as the film on a diamond coated tool, e.g., a cutting tool, the cutting performance and the tool life may vary due to the different thicknesses of the diamond film at the respective corners of the cutting tip of the tool. Therefore, the tool cannot be used for stably cutting workpieces.
Therefore, it is an object of the present invention to provide a microwave plasma CVD apparatus which is capable of two-dimensionally adjusting the position of a plasma in a reaction tube.
Another object of the present invention is to provide a method of synthesizing a diamond film of uniform quality and thickness on a substrate with the above microwave plasma CVD apparatus.